Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PURIFICATION OF WASTE GAS
HAVING A HIGH CHLORIDE CONTENT
The present invention relates to a process for the
purification of waste gas (flue gas) having a high
chloride content, and more specifically of a flue gas
from an incineration plant by a multi-step absorption of
the contaminations, wherein in a ~irst step the strongly
acidic components of the waste gas such as HCl and HF
are removed by washing with water which first contains
up to 20 g/l of HCl, and in subsequent steps the less
acidic components of the waste gas such as SO2 and NOX
are removed. --
In the purification of flue gases, HCl and HF play
a big part, besides SO2 and NOX, and especially so, if
the flue gas is derived from a waste incineration plant.
Depending on the composition of the garbage subjected to
incinération, the HCl content~of the flue gas may amount
to even ~more than 10 gjm~. As a rule, such high HCl
contents require a particular treatment in ~lue gas
purification plants. Today this treatment is mostly
effected by a step of HCl removal preceding the step of
52 removal. Therein, the HCl removal by deposition is - :
often carried out simultaneously with an operation of
quenching the flue gas wherein freq~lently alkaline
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chemicals are already added. Furthermore, in this
quenching step a major part of the residual dust having
remained in the flue gas is deposited.
The solution or suspension obtained in said
depositing procedure, which mostly is still acidic due
to hydrochloric acid present, in general contains the
largest portion of the heavy metals contained in the
flue gas. Said solution or suspension containing hydro-
chloric acid is mostly subjected to a waste water treat-
ment. Upon the addition of an alkali or o~ alkaline
earth carbonates or hydroxides, neutralization occurs
along with a precipitation of the heavy metals. After
the filtration of the suspension, the filtrate is
usually passed to an outfall ditch or evaporated with
the residue being conveyed to a dumping ground. If the
waste water is entered into an outfall ditch, it mostly
contains still higher amounts of soluble salts and some
residual contents of heavy metal.
If it is intended to recover utilizable salts such
as common salt or calcium chloride from said residues,
an efficient and expensive procedure of waste water re-
processing must generally be carried out prior to the
evaporation and, depending on the circumstances, the
crystal mass precipitated in the course of the evaporat-
ion must be subjected to one or more recrystallization
steps which involve a high apparative and energetic
input while, nevertheless, it generally doe~ not yet
ensure the degrees of purity for the salts to be obtain-
ed as demanded in practice.
It is the object of the present invention to
further improve the process, and render it easier and
safer, for the purification of flue gas having a high
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chloride content, and especially of flue gas from an
incineration plant, so that amounts as large as possible
of re-usable products are recovered and only minor
amounts of waste materials burdening the waste water
effluents or the dumping ground environment are pro-
duced. More particularly, it is the object of the
invention to conduct the removal of the stongly acidic
components such as HCl and HF from the waste gas in a
manner so that re-usable substances reliably and
ine~pensively are recovered therefrom. The process is
based on those processes wherein in a first step the
strongly acidic components of the waste gas such as HCl
and HF are removed by washing with water which first
contains up to 20 g/l of HCl, and in subsequent steps
the less acidic components of the waste gas such as S02
and N0x are removed.
Said object can be attained in a surprisingly easy
way by that already in the first step the hydrochloric
acid is concentrated to a level in excess of 50 g/l of
HCl, and preferably in excess of 80 g/l of HCl, and the
diluted hydrochloric acid thus obtained is separated by
rectification into a concentrated hydrochloric acid and
a bottoms fraction.
It has been surprisingly found that in this way it
is possible in particular to obtain an azeotropic
hydrochloric acid of about 22% by weight which, in
addition, is almost free of impurities and particularly
of heavy metals. This result was especially surprising
because in the acids obtained by the concentrating step
there are still present larger amounts of heavy metals
and other salts solu~le in water and hydrochloric acid
and nevertheless, upon rectification, these materials
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remain in the bottoms fraction. This was not foresée-
able since it has been known that in the gases from
garbage incineration plants there are present also metal
chlorides that are steam-volatile. Furthermore, it has
been surprisingly found that the solids and salts
accumulated in the rectification bottoms such as, e.g.,
calcium s~lfate, do not produce any build-up and, hence,
do not cause any malfunctions. In conventional
evaporators for waste waters containing chloride salts,
such build-up causes frequent failures and longer
periods of breakdown to occur.
If in the past in a first step the strongly acidic
flue gas components such as HCl and HF were separated
off by washing with water alone while not with the
addition of alkaline reagents, the concentration of HCl
was allowed to be 20 g/l of HCl as the maximum, because
otherwise to larye amounts of HCl would be left in the
vent gas. However, a hydrochloric acid having been that
much diluted could by no means be processed to give a
more concentrated hydrochloric acid for economic
reasons. Thus, according to the invention, this diluted
hydrochloric acid containing up to 20 g/l of HCl is
subjected to a step of concentrating same to more than
50 g/l of HCl.
It is preferred that the concentrating step leads
to a level within the range of from 80 to 130 g/l of
HCl. Depending on the HCl content of the flue gas,
higher levels to be obtained in the concentrating step
are possible. For example, this concentrating operation
in the first step may be effected by that the HCl
washing is continuously done in the counter-current
whereupon a concentrated hydrochloric acid is already
obtained.
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~ concentrating operation carried out in a discon-
tinuous mode by counter~current has proven to be more
advantageous. In this mode of operation, t~e flue gas
to be purified is passed ~hrough at least two washing
stages; therein it is first contacted with batches of a
more diluted hydrochloric acid the concentration of
which is increased, whereafter it is subjected to
washing with fresh water batches which allowed to become
concentrated to a level of up to 20 g/l of HCl. As soon
as this critical limit of the residual HCl content of
the flue gas will have been reached, the batch will be
replacd by fresh water. The hydrochloric acid having
been concentrated to at least 50 g/l of HCl is taken out
and replaced by the more diluted HCl washing solution
containing a maximum of 20 g/l of HCl.
Above all, this mode of operation is to be carried
out under easier conditions with respect to the required
equipment. Changing batches may be readily controlled
by monitoring the chloride content in the washing
solutions. To render the process flexible, it lends
itself to provide intermediate reservoirs for the
various fractions of the washing solutions.
A further preferred embodiments of the process
consists of causing the concentrating procedure to take
place in a quencher wherein the flue gas is cooled to
the limiting temperature o~ cooling and the residual
dust is removed. In the bottoms of such a quencher, the
HCl content is readily increased to at least 50 g/l of
HCl. Said quencher bottom product can be directly
introduced into the rectification plant, if desired or
required after the removal of solids therefrom.
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Steam is removed from the head of the rectification
unit, which steam may be recycled into the waste gas
purification plant at any optional location. From the
bottoms there is withdrawn a fraction not tending to
build-up formation and having a HCl content in response
to the rectification pressure. Furthermore, the
azeotropic mixture corresponding to the rectification
pressure of HCl and water is withdrawn from the recti-
fication column. If the rectification is effected under
normal pressure, an azeotropic hydrochloric acid of
about 22% by weight of HCl is obtained. If it is
desired to produce a hydrochloric acid having a higher
concentration, the rectification pressure must be
reduced. If higher pressures are employed, an azeo-
tropic mixture of HCl and water is produced which, when
condensed, contains less than 22% by weight of HCl.
Upon the rectification at normal pressure according
to the invention, first an azeotropic hydrochloric acid
of about 22% by weight of HCl is obtained, which
contains only traces of heavy metals such as mercury and
has not been contaminated by HF; HF as present obviously
undergoes a reaction with the always present calcium and
silicon compounds already during the concentrating and,
at the latest, rectifying steps and, thus, is discharged
from the process together with the solids from the
guencher bottoms and/or bottoms of the rectification.
In any event, according to the invention first an
azeotropic hydrochloric acid corresponding to the
respective rectification pressure is obtained, which is
free of interfering impurities and, thus, can be used
for various purposes. I~ desired, it may of course also
be used to neutralize sufficiently pure alkali or
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alkaline earth carbonates and/or hydroxides and there-
from to recover commercially utilizable clean salts
which at least do not comprise the high contents of
heavy metals as so far common.
Also the bottoms fraction of the rectification can
be utilized in several ways according to the invention.
Thus, it is possible, for example, optionally after
separation from the solids, to recycle part of the
bottoms fraction to the concentrating stage, all the
more since it still contains considerable amounts of
HCl. It is also possible to jet part of this bottom
fraction of the rectification into the flue gas stream
prior to the dust deposition. There, the solid compo-
nents are deposited together with the dust, while the
HCl gas and the water are recycled into the process.
Furthermore, it is possible to pass part of the bottom
fraction of the rectification to the fly ashes process-
ing step and thereby to remove it from the process. The
fly ashes in general contain alkaline components in an
amount sufficient to neutralize the hydrochloric acid
contained in the bottom product and, thus, also to
render harmless that proportion of H~l discharged from
the process.
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Modern purification units for flue gases from
garbage incineration plants to an increasing extent have
been equipped with additional condensation filters or
wet-E filters. The waste water from said filters can be
employed as the absorbent for HCl and HF in the first
stage, so that also here no waste materials are formed
any more that are undesirable or give rise to problems.
Thus, the absorption with water of the strongly
acidic components of the waste water, i.e. HF and HCl,
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is followed, optionally after a purification through
condensation filters or wet-E filters, by the convent-
ional removal of S02 and NOx. More particularly, for
the desulfurization, Applicants' two-cycle process using
limestone has proven to be useful. Also the subsequent
removal of NOx can be realized by way of any convention-
al and so far known procedure.
The water and energy balances of the process
according to the invention show that concentrating the
hydrochloric acid and subsequent rectification to form
concentrated hydrochloric acid does not cause any
noticeable increase in the water consumption or energy
consumption. The steam formed in the concentrating and
rectifying steps is anyway needed to condition the gas
to be purified for the desulfurization plant.
Discharged from the process is only the water content of
the more concentrated azeotropic hydrochloric acid.
Furthermore, solids are produced which in the same
manner as previously are taken out from the dust
separator, quencher bottoms and the effluent water from
the condensation or wet-E filters. According to the
invention, also these fractions can be treated,
processed or put into further use either in the same
manner as before or in a slightly modified manner. From
the process according to the invention, there is
additionally recovered a clean and readily usable
concentrated hydrochloric acid, so that the elimination
of chlorides as so~far practiced becomes dispensable.
Nevertheless, for some intended uses a concentrat-
ion of higher than 22% of the azeotropic mixture of
hydrochloric acid is re~uired. According to the
invention, also such a hydrochloric acid of a higher
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concentration can be produced by subjecting the flue
gas, after the quenching procedure, to a multi-stage
discontinuous, with respect to the liquid phase,
absorption, wherein in a first step following the
quenching step the hydrochloric acid is concentrated to
the desired final concentration, while the fine-cleaning
of the flue gas is effected only in the last absorption
step.
In the course thereof, the dust-loaded flue gas is
subjected to three steps of wet washing. The stage for
the first step is designed as a concurrent spray absorb-
er. First, the flue gas is quenched at several spray
levels and liberated from dust and heavy metal com-
pounds. The washing agent is recirculated. Evaporated
liquid is intermittently or continuously passed to the
quencher bottoms. B~ a time-controlled discharge of
liquid from the quencher bottoms, the salt concentration
in the washing agent is limited to a predetermined
value.
In the described first step~ there is adjusted a
HCl concentration which is in an eguilibrium with the
HCl content of the flue gas. Now, the flue gas, via a
stack tray, passes into the second stage, and therefrom
optionally in to the further stages, of the absorber,
which stages are preferred to have been designed as
counter-current washer with fillings. In each stage the
water is recirculated. In this process, the first stage
behind the quencher acts as a concentrating stage, while
in the following stages, and particularly in the last
stage, the fine-cleaning is effected. If the desired
values of the purified gas at the outlet of the last
absorber is reached, the fine-purification stage is not
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capable of absorbing any further HCl, because otherwise
the values of the purified gas would increase. It is
not later than at this time that the concentrated acid
is removed from the last stage into the receiver of the
previous step wherein it may be further concentrated.
The last purification stage or fine-purification, after
changing the liquid, is charged with fresh water or
waste water obtained from the total process.
In the design as described, the concentration in
the penultimate stage is highly dependent on the demand-
ed HCl level in the pure gas. If the demanded HCl level
in the pure gas is desired to be high, then it takes a
relatively long time until the equilibrium concentration
related to the pure gas level has been established in
the liquid phase. For an accordingly period of time the
concentration may be carried out in the preceding step,
where the maximum concentration value may be reached
which is in equilibrium with the HCl partial pressure at
the inlet of the penultimate stage.
However, since in most cases the levels demanded
for pure gas are very Iow (< 5 to lO mg/m3), the equi-
librium concentration will not be obtained through three
stages only. Thus, in order to obtain a highly concen-
tjrated hydrochlorid acid, on the one hand, and a low
pure gas level, on the other hand, it is generally
necessary, to operate in four absorption stages alto-
gether.
An expedient variant of the process according to
the invention utilizes the amount of water required for
quenching for the fine-purification of the flue gas. To
this end, said amount of water is added in the last
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stage and there absorbs the remaining hydrochloric acid.
The resulting very diluted hydrochloric acid is then
directly passed into the quenching stage. This arrange-
ment allows for an extended period of time to pass until
the liquid must be changed, so that in the median stages
a higher concentration can be attained.
A further advantageous embodiment of the process
according to the invention consists of turning off the
liquid supply in the quenching step a definite time
before reaching the pure gas level at the outlet of the
last absorber. The hot flue gas causes part of the
quencher liquid to evaporate, so that the hydrochloric
acid contained therein is released to a considerable
extent into the gaseous state. Thus, at the outlet of
this stage the HCl partial pressure is increased whereby
also the HCl concentration in the following concentrat-
ing step is increased.
During the time of the HCl evaporation in the
quenching step, the washing cycle of the fine purificat-
ion stage is supplied with the amount of water provided
therefor, so that the pump reservoir thereof must only
have the size as appropriate therefor. ~ The HCl
evaporation in the quencher may be continued until the
desired level in the purified gas after the fine-
purification is reached. Then, upon change of the
liquid, the stored proportion of~the liquid is added to
the quencher bottoms. More particularly, if the flue
gas enters into the quenching stage after already having
been cooled to a very high degree, the amount of water
required for quenching is reduced. Then it will make
sense to employ the described procedure of HCl evaporat-
ion and to extend the total process by one more step.
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Then it is nevertheless possible to reach the demanded
high HCl concentration in the concentrating step at the
same low HCl pure gas level. Thus, it is well possible,
through several steps to obtain a highly concentrated
hydrochloric acid and, at the same time, to retain low
HCl levels in the purified gas even from relatively much
cooled flue gases.
The process according to the invention may be
controlled by measuring the HCl levels in the purified
gas. As soon as the demanded HCl levels in the purified
gas has been reached, the change of liquids from the
following steps to the preceding steps, respectively, as
described is carried out.
The process according to the invention is further
illustrated by the following Example run under aggravat-
ed conditions of operation. -
E X A M P L E
In a garbage incineration plant, flue gases con-
taining about 1.5 g/m3 of HCl after the dust separator
are fed to a quencher supplied with industrial water.
The residual fine dust is collected in said quencher.
Furthermore, in the quencher bottoms, due to the
evaporation of water, the HCl concentration rises to
about from 60 to 70 g/l of HCl. The quencher bottoms,
after the separation therefrom of the solids by means of
a centrifuge, is introduced into a rectification plant,
from which steam is withdrawn at the head, and a suspen-
sion free from build-up is withdrawn from the bottom.
Approximately at a height of on third from the bottom of
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the rectification column, the azeotropic mixture of HCl
and water is withdrawn. After condensation it forms an
uncolored hydrochloric acid having a content of 22% by
weight of HCl. The contents of mercury and other heavy
metals are below the limits of detection or within the
ranges of a customary, commercially available azeotropic
hydrochloric acid. The flue gas stream of the inciner-
ation plant is passed from the quencher to a second
washing stage, wherein it is accomplished that the
contents of hydrochloric acid does not exceed 20 g/l of
HCl. Thereby it is ensured that the vent gas stream
exiting from this stage only comprises the admissible
HCl content. The diluted hydrochloric acid from this
washing stage is introduced into the quencher.
Downstream of the described chloride washing stage
there is located a wet-E filter. ~rom there, the vent
gas stream is advanced into a two-cycle absorption
washer with limestone and desulfurized. The waste water
from the wet-E filter is passed to the washing water of
the chlo~ide washing.
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